A wide variety of bearings for rotating machinery, ranging from conventional bearings to noncontact bearings, are available. Conventional bearings, in which bearings physically contact a rotating device, are subject to many, well known problems. These problems include frictional energy losses and mechanical wear.
Noncontact bearings, such as magnet bearings, overcome problems with friction and mechanical wear, but introduce other problems. For example, permanent magnet bearings are inherently unstable. As a result, they require external mechanical means to stabilize them in at least one degree of freedom. Electromagnet bearings, on the other hand, can be made inherently stable with position sensors and electronic feedback control loops. The electromagnets in the bearings, however, require a power source and a means for cooling their windings. As a result, electromagnet bearings can be impractical for many applications.
Superconductor bearings have been proposed as improvements to permanent magnet and electromagnet bearings. Early superconductor levitation experiments incorporated Type I superconductors, which are perfectly diamagnetic in their superco states. This means that external magnetic fields (H) less than a critical field (H.sub.c) that are applied to Type I superconductors induce magnetic fields in the superco that are exactly opposite to the external fields (H&lt;H.sub.c). For example, an external magnetic field poled north-south (N-S) induces a field poled S-N that repels the external field. Similar behavior can be observed in Type II superconductors that are exposed to external magnetic fields less than a lower critical field (H.sub.c1)(H&lt;H.sub.c1). External magnetic fields between H.sub.c1 and an upper critical field (H.sub.c2) can also induce a measurable, though diminished, repulsive force in Type II superconductors (H.sub.c3 &lt;H&lt;H.sub.c2).
Type I superconductor bearings use the repulsive force between the oppositely poled magnetic fields to levitate a magnet that is part of a rotating shaft. Because all known Type I superconductors are superconductors only below about 21 K, they require liquid helium for cooling. The drawbacks of liquid helium cooling, such as cost, are well known.
Recently, bearings with Type II superconductors have been proposed. Type II superconductors can generate larger induced magnetic fields than Type I superconductors because they have larger critical fields than Type I superconductors. Therefore, Type II superconductors are potentially more useful in bearings than Type I superconductors. U.S. Pat. Nos. 4,886,778 and 4,939,120, both to Moon et al., describe several Type II superconductor bearings. The Moon et al. bearings use the repulsive force between a Type II superconductor and an applied magnetic field to levitate a magnet that is part of a rotating shaft. FIG. 1 shows how these bearings work with an applied magnetic field below H.sub.c1 (H&lt;H.sub.c1). A magnet 2 applies a magnetic field, represented by dashed lines 4, to a superconductor 6. The magnetic field 4 induces surface screening currents, represented by arrows 8, in the superconductor 6. The surface screening currents 8 preduce a magnetic field opposite to the applied field 4. This is indicated by the N-S poling on the magnet 2 and S-N poling on the superconductor 6. The opposite fields repel each other and produce a force F.sub.r that levitates magnet 2 to support the force L. Unlike Type I superconductors, the repulsive force F.sub.r in Type II superconductors can be offset by an attractive force F.sub.a, shown in FIG. 2, that can be induced in the superconductor by trapped magnetic fields. The net force, F.sub.r -F.sub.a, exerted by a Type II superconductor bearing depends on the way the superconductor was cooled to its superconducting state.
A Type II superconductor can be cooled to its superconducting state under a zero field cooled (ZFC) protocol or a field cooled (FC) protocol. Under a ZFC protocol, the superconductor is cooled without being exposed to a magnetic field. When an external magnetic field is later applied to the ZFC superconductor, the applied field generates the surface screening currents and repulsive force shown in FIG. 1. (H&lt;H.sub.c1). If the applied field is smaller than H.sub.c1, the ZFC superconductor behaves like a Type I superconductor. If the applied field is larger than H.sub.c1, a magnetic flux poled parallel to the applied field penetrates the superconductor in the form of fluxons (H.sub.c1 &lt;H&lt;H.sub.c2). The fluxons are represented by arrows 10, which are circulating supercurrents that support the magnetic field contained in the fluxons. For example, an applied field that is poled N-S, as shown, produces fluxons that are also poled N-S. As a result, there is an attractive force F.sub.a between the fluxons and applied field that offsets the repulsive force F.sub.r. If the superconductor has a high density of strong pinning centers, the fluxons will be pinned near the surface of the superconductor. Pinning centers are defects in the superconductor capable of pinning fluxons in a particular position. The fluxons pinned at the surface of the superconductor inhibit other fluxons from entering the superconductor. As a result, the attractive force F.sub.a in a ZFC superconductor can be small compared to the repulsive force F.sub.r at field strengths above H.sub.c1. Therefore, the net force, F.sub.r -F.sub.a, in a ZFC superconductor can be strong enough to support a load at most field strengths below H.sub.c2.
Under the field cooled (FC) protocol, the superconductor is cooled in the presence of an applied magnetic field. During cooling, some of the field is trapped as fluxons at pinning centers in the superconductor. This creates a situation similar to the one shown in FIG. 2 For a FC superconductor, though, the FIG. 2 situation prevails for all applied magnetic fields below H.sub.c2 rather than just between H.sub.c1 and H.sub.c2 as with a ZFC superconductor. The fluxons trapped during a FC protocol are dispersed throughout the superconductor rather than trapped at the surface as in a ZFC protocol. The trapped fluxons produce a field density that is nearly equal in magnitude, but opposite in sign, to that produced by the surface screening currents. As a result, the net magnetization on a FC superconductor can be close to zero and the attractive force F.sub.a can substantially offset the repulsive force F.sub.r. Therefore, the net force, F.sub.r -F.sub.a, in a FC superconductor can be too small to support a significant load.
Because the net force generated by FC superconductors can be low, the Moon et al. bearings should be built with ZFC superconductors. ZFC superconductors, however, can be impractical for many applications because they require the superconductors to be cooled without being exposed to magnetic fields. For example, if a Moon et al. bearing uses permanent magnets, the superconductors in the bearings must be shielded from the magnets during cooling. This can be done by removing the magnets from the bearing while the superconductors are cooled and later replacing them. Such a procedure, though, is impractical for most applications. If a Moon et al. bearing has electromagnets rather that permanent magnets, the electromagnets can be shut off, rather than removed, during cooling. Electromagnets, however, require a power source and means for cooling the magnets' coils. These requirements can make Moon et al. bearings that have electromagnets less desirable as well.
Therefore, what is needed is a Type II superconductor bearing that can be cooled to its superconducting state in the presence of an external magnetic field.